1. Field of Invention
This invention pertains in general to the field of shaft sealing devices, and more particularly to a static and dynamic shaft sealing arrangement. Even more particularly, the present invention pertains to an expeller shaft sealing, which effectively seals statically when a shaft is at rest and which effectively seals dynamically when the shaft is rotating, as well as during transitions between static and dynamic operation. A sealing arrangement changes configuration so as to seal effectively statically by means of a mechanical contact at a sealing surface and without friction when the shaft is rotating, and whereby the sealing arrangement provides a good sealing effect even for a differential pressure in the surrounding media, such as liquid, gas or dust, between both sides of the sealing arrangement both in the static and dynamic operation.
2. Description of the Prior Art
Today, lip seals are mainly used for isolating bearings in rotating equipment. The seals and bearings account for a large number of rotating equipment failures and there is a close relationship between the lives of these two critical components. The failure of such a seal may cause the bearings to fail and poor bearing conditions can reduce seal life. Rain, product leakage, debris, and wash-down water entering the bearing housing contaminate the bearing lubricant and have a devastating effect on the product lifetime of the bearing. Very small amounts of water or other contaminants can shorten bearing life considerably.
Auxiliary mechanical equipment shaft sealing devices, sometimes called bearing isolators or sealing rings, are used for equipment, which is intended to operate in hostile applications, in which the equipment is exposed to potential contaminants as dust for instance. Elastomeric shaft seals thus quickly wear out and fail in such hostile environments. Dust and other exterior contaminants cannot be excluded from the interior of a sealed housing by a failed standard sealing device. Oil or other fluids can neither be prevented from leaking out of the transmission devices past a worn lip seal. It is not possible to prevent the ingress of contaminants and the egress of lubricating fluids when a differential pressure exists in the sealing devices' surrounding media, such as liquids, gas or dust, between both sides of the sealing device. In both static and dynamic operation, a differential pressure contributes to a leakage of the known seals and supports the transport of contaminants over the barrier of the seals.
An example for a static and dynamic shaft seal assembly is disclosed in U.S. Pat. No. 5,221,095, wherein a solid, circumferentially stretchable annular seal member is mounted on a rotor female surface and engages a stator male surface when the rotor and seal member are at rest. The deformable sealing member is stretched circumferentially in radial direction by centrifugal force out of engagement with the stator when the rotor and seal member are moving at operating speeds, thus eliminating friction of the seal member.
However, although this seal assembly offers protection against rain, product leakage, debris, and wash-down water entering the bearing housing, the disclosed seal assembly does not seal when a pressure difference exists over the shaft seal assembly. The pressure difference may be caused by e.g. a pump effect on the bearing side or by an overpressure on the exterior side. For instance, such an overpressure on the exterior side of the sealing assembly is caused e.g. by cleaning equipment such as high-pressure wash appliances, or if the housing is positioned under water which causes an increased in exterior pressure due to the column of water existing above the housing. The differential pressure may also be generated by temperature variations, e.g. caused by exposure to heat from the sun during the day and cooling during the night, or by heat generated inside the housing by e.g. friction or power dissipation of driving devices. When heated, fluid inside the housing expands and an increased pressure results and vice versa. Such differential pressures cause the known sealing members to be lifted away and to loosen out of mechanical contact with the adjacent sealing surface, which results in a loss of sealing giving way to a passage for contaminants to e.g. a bearing and thus shortening of the product life of the equipment comprising the sealed shaft.
Moreover, the seal assembly disclosed in U.S. Pat. No. 5,221,095 is difficult to assemble as the elastic sealing member has to be positioned against its contracting elasticity into the sealing assembly.
Another shaft sealing assembly is disclosed in CH-369329, wherein an O-ring statically seals a shaft. The O-ring is located in a rotor recess having coaxial walls with a certain inclination angle relative to a radially oriented stator. In this way, the O-ring is by means of its elasticity pressed against a radial stator surface and a sealing effect is achieved. On rotation of the shaft, the O-ring is caused to circumferentially expand due to the centrifugal force experienced. By means of one of the inclined circumferential walls, the O-ring travels further axially and radially away from the stator. Thus, contact friction of the O-ring is eliminated upon rotation of the shaft. This shaft seal assembly is easier to assemble than the previously described assembly disclosed in U.S. Pat. No. 5,221,095. However, this shaft sealing assembly does similarly suffer from the disadvantage that the sealing assembly does not seal when a differential pressure exists in the surrounding media on the two sides of the shaft seal assembly.
Thus, the problem to be solved is to provide a new shaft seal assembly insensitive to differential pressures in the media on both sides of the sealing assembly, ensuring protection against ingress of contaminants and egress of lubricants, both in static and dynamic mode of operation.
Another problem to be solved by the invention is to provide a machinery seal of the type described above, in which a solid sealing member engages both a seal stator and a seal rotor when the shaft is at rest, and in which the sealing member expands away from the stator when the shaft rotates.
Still another problem to be solved by the invention is to provide a seal of the type described above, which provides easy assembly, manufacture and a long product life cycle.
Yet a further problem to be solved by the present invention is to provide a sealing for rotating shafts with large diameters up to approximately 3 m, such as approximately 1 m. Shafts with such large diameters requiring effective static and dynamic sealing are for instance used in water driven turbines in hydro power plants or in propeller shaft sealings of vessels.
Furthermore, the person skilled in the art will be able to identify further problems associated with the prior art, which are not explicitly stated in the text of this application, but which are solved by the present invention.